Beomaster 901 hum on left channel

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beginswithaJ Posted: Sat, Aug 4 2018 3:13 PM

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Hi Guys, 

Love this forum and hope you can help me on my first B&O restoration :) 

I came across a semi-working 901 which has a 50hz hum in it's left channel. And I'm not giving up on this puppy YET! :D I'm not a pro electrotechnician, but I do know how to hold a soldering iron (the cold end, right?!), how to read a schematic and what a transistor does :P 

So far I've been able to determine:

• hum only appears on left channel 
• hum is heard on speakers
• hum is heard on headphones too
• hum is NOT heard on tape out

• switching to another input doesn't matter. still hums.
• the humming is about as loud on volume setting 0 as it is on 10
• the humming is same volume on any balance setting except on right-1 to right-2, then the humming gets REALLY loud on the left channel
• when the amp is switched on for a few seconds the hum starts sounding different... sharper is the best description I think. Less of  a sine, more of a clipped, tangent or square wave. 

(unrelated) fixes I already did:

• all caps have been replaced
• the darlingtons have been replaced as well
• input switches, slider pots and volume pot have been cleaned (just for the sake of complete info) 

So my guess is that it's at least confined to the left output stage, but I don't really know what to try next.

Do any of you have an idea where to start looking for more issues, or how to measure for them? (I do have a multimeter, but don't own something fancy like a scope ;)

Thanks so much!!

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Craig replied on Sat, Aug 4 2018 3:31 PM

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I had something similar on a BM3000 recently, turned out to be a noisy transistor...I found it the hard way (by swapping each transistor from the left channel to the right one at a time until the noise switched channels therefore identifying the culprit) I was advised after the event that a quicker way is to apply a freezing spray to each transistor in the rogue channel one at a time and seeing which one is responsible the easy way....haven't had to try this but hey......got to be worth a shot!

Craig

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Dillen replied on Sat, Aug 4 2018 3:33 PM

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Why did you replace the darlingtons? And what types did you fit?
Did you replace the idle current trimmers and could you set the idle currents correctly?

Martin

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beginswithaJ replied on Tue, Aug 7 2018 4:14 PM

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Craig:

I had something similar on a BM3000 recently, turned out to be a noisy transistor...I found it the hard way (by swapping each transistor from the left channel to the right one at a time until the noise switched channels therefore identifying the culprit)

Craig

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beginswithaJ replied on Tue, Aug 7 2018 4:41 PM

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Dillen:

Why did you replace the darlingtons? And what types did you fit?

The reason was that the right channel was already retrofitted by someone else with a set of Tip120 and 125 darlingtons, while the left channel still had the original Motorola TE's. I thought maybe they could be defective, desoldered them and measured them. Got some wierd values. So I replaced all four with standard Tip120 and 125 to be sure since they cost next to nothing and seem to work on the right channel before this whole operation so... Later I discovered that these Motorola's don't measure like standard NPN's so they weren't faulty to begin with *facepalm* Oh well.. couldn't hurt anyway. 

Dillen:

Did you replace the idle current trimmers and could you set the idle currents correctly?

Trimmers have not been replaced yet, but I don't actually know how to measure the idle currents yet seems fiddly. I'm reading up on this as we speak..

But in the meantime: how could an offset idle current be responsible for this humming? Just out of curiosity, I'm trying to learn here :)

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Dillen replied on Tue, Aug 7 2018 5:40 PM

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High idle current=heavy load on power supply.

This ups the ripple and you get hum (and heat).

Martin